Earth induction compass



June 1948. CARL-ERIK GRANQVIST 2,444,290

EARTH INDUCTION COMPASS Filed Nov. 27, 1

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Pans: SAl/PTEI? INVEN TOR. 6:161 fem fem 9001 A TTORHE Y.

Patented June 29, 1948 I EARTH INDUCTION COMPASS Carl-Erik Granqvist, Lidingo, Sweden, assignor to Svenska Aktiebolaget Gasaceumulator, Lidingo, near Stockholm, Sweden, a corporation,

of Sweden Application November 27, 1943, Serial No. 512,060 In Sweden February 2, 1943 3 Claims. (Cl. 33-204) This invention relates to earth induction compasses and has for an object to provide a compass of the above type having novel and improved details of construction and operating characteristics.

The invention relates more particularly to earth induction compasses and similar navigation instruments of the type having magnetic cores of high permeability which are adjustable in azimuth. The cores comprise a pair of relatively fixed members and an intermediate rotatable member which is adapted to vary the reluctance of the magnetic circuit at a uniform rate. The magnetic circuit is polarized by an alternating field. The relatively fixed member is surrounded by output windings so arranged that voltages are induced therein having a component due to the alternating current excitation modulated by the alternating voltage due to the horizontal component of the earths field.u

A. further object of the invention is to eliminate the efiect of disturbing alternating current components which may be generated in the system and thereby. improve the accuracy of the result.

In accordance with the present invention, this is accomplished by using an alternating current watt meter having coils one of which is connected to the source of alternating exciting current and the other of which is energized as a function of the induced potential derived from, the output windings. This potential may be rectified and amplified before being applied to the watt meter. The arrangement is such that the watt meter is influenced only by currents of the predetermined frequency and the effect of all extraneous influences which tend to produce currents of 3 other frequencies is eliminated.

The watt meter will give an indication whichis a function of the angle between the longitudinal axis of the system of iron cores and the direction of the horizontal component of the earth magnetic field. It is possible to read this angle either directly on the watt meter or by turning the iron cores in the direction in which the watt meter gives a zero indication. I

The invention is further described in connection with the annexed drawing, in which Fig. 1 is a schematic diagram illustratinga simple form of the invention, Fig.2 is a diagram for explanation of the operation of the invention, Fig. 3 is a detail view of the dial of the watt meter, and

Fig. 4 is a schematic diagram illustrating the invention.

In the arrangement according to Fig. 1, the magnetic circuit contains the two fixed magnetic cores l and II and a rotatable magnetic core l2 placed between them. The latter one is mounted on a shaft l3, which is Journalled at M and I5.

The cores l0 and II further are surrounded each by one winding l8 and I1, respectively, said two windings being connected in series. Their free terminals are connected to a rectifier l8, and a resistor I9 which is by-passed for alternating current of higher frequencies by means of a condenser 20.

The cathode of an amplifier tube H is connected to one terminal of the resistor I9 through a cathode bias resistor 22, its control grid being connected with the other terminal of resistor I9.

The cathode bias resistor 22 is by-passed for alternating current by a condenser 23. The anode circuit of the tube 2! contains the primary winding 24 of a low frequency transformer, tuned to the modulation frequency by means of a condenser 25, the secondary winding 26 being combined with one coil 21 of a watt meter working on a dial 28. The other coil 29 of the instrument is connected with a line 30 carrying alternating current of a fixed frequency. a

The line 30 is also connected to a coil 3! in series with a phase correction filter 32. The coil at is associated with the magnetic cores l0--ll in such a way that the field created by the coil causes an alternating polarizing action. The phase correcting filter 32 may contain an ohmic resistance.

The above described arrangement according to Fig. 1 acts in the following manner:

It is assumed, that theshaft l3 rotates at /20! revolutions per second, the line 30 having a frequency of in: periods a second. The horizontal component of the earths magnetic field is represented by H11, the angle of this field with thelongitudinal axis of the magnet system Ill-ll by a, and the amplitude of the intensity of the field created by the coil 3| by Hm. The rotor part It causes an alternating field 4 to pass through the magnetic circuit, the magnitude of which is in which formula K1 represents a constant determined by the properties of the magnet system.

' 4 The expression of o is developed into the form: of a cosine curve, which has the form in a polar cos a 005 MM? diagram of a figure l as shown in Fig. 2. An

investigation of the form of the curve shows, that l maximum reading is obtained in the two direc- "2 [Sm "f m (2) tions N and 8; whereas the reading should be For determining the voltage generated in the zero in the two geographical directions E and W. winding l'8l1 the expression oi i is difl'eren In practice it is, of course, not always suitable tiated: to use the instrument as a direct-reading com- As will be shown below it is oi substantial impass because there are a number of sources of portance that or: be small in relation to m. Hence errors. For examplethe fiat form of the curve 'as a first approximation the factors (nn-l-m) and according to Fig. 2 adjacent to north and south (w1w2) may be replaced with or. The Equamakes the device insensitiveinthisposition. Also tion 3 then will take the following form: the reading or the instrument is not dependent From this the voltage in the winding It--i'l solely upon the above mentioned geographical is derived as: angle, but also depends upon the field intensity In Equation 5, K2 is a constant determined by of the horizontal component of the earth's mag- K1, the constants of the windings, o1 etc. netism provided that the instruments always are From. Formula 5 it is evident that the carrier placed in horizontal position. The instrument wave on is modulated to produce two sidehands may be held in a horizontal position by mountdisplaced by the frequency on from the carrier ing a gyroscope G with the rotating system i2-I3 wave. 7 so as to remove the last named source-of error.

It is the voltage 0 that is rectified in. the recti- On the other hand the source of error due to her l8. To begin with it is assumed, that this the horizontal component of the earths magrectification takes place in the usual manner by netism varying in intensity from place to place amplifying the voltage on a square characteristic, on the earth cannot be thus eliminated. Hence so that each of the square terms of the contained it is necessary to know the field intensity of the parts and also the double product of these two horizontal component of the earthsmagnetism parts are obtained. The square alternating-voltat the place where the instrument is used, and

age terms in the rectifier output will be of a high the dial of the instrument must be arranged in frequency so that they give no voltage across the such a. way that the direction may be read at difresistor l9, as the condenser 20 shortcircuits this ferent values of field intensity. Such a dial may resistor for alternating current of higher fretake the form of a diagram, as shown in Fig. 3, quencies. On the other hand, the double prodbut it is evident that such a dial may be diflicult uct of the terms represents a rectified voltage: to read by a person not skilled in more compli- 1 cated instrument readings. 2H,. cos a sin (anti-H [cos (m +w )t cos (cor-co On the dial according to Fig, 3 the difl'erent 4 (6) concentric circles represent the diiierent values of the horizontal component of the field intensity, After development the expression 6 will give: 5 and the bent radial curves indicate the gem a hi al directions. HkHm it? slfould be mentioned that the line E-w in 1 (7) Fig. 3 indicates the zero-value of the instbrgment, The first and the last part of expression 7 as the reading in this direction should zero' also represent high frequencies which will be byas is evident n the two directions passed by means of the condenser 23. Across the g gg Z; f gfi g gg gfig gg gg resistor 22 therefore a. voltage will be developed. reading is not present in these two directions. L= a n n cos a 5111 This characteristic may be utilized by arranging in which equation K1 is a constant determined by the magnet cores Ill-ll manually m K: and by the roperties of the resistor is and i ly t ether witha lin ar raded c mpass ard t rectifier g, on which the geographical direction may be read. The voltage E1. is fed to one winding of 'the watt wh n using the inst ument in he l t m ntion d meter the other winding of which is fed directly m n i is On y n c ary to place the longifrom t lines 30 th a voltage The filter tudinal direction of the cores Ill-ll so that the 32 is assumed to be so dimensioned that both of t meter shows e o i dic tio he s ethe voltages are in phase. The torque in the watt r phi l dir ti n is then determin d s th meter then is obtained as: gngle glhich the 'cores :}0 llirave been tux-neg rom eir direc ion a zero dication whic V "=K F cos i (9) 7 must always be the'magnetical east-west direc- F'rom Equation 9 it is evident that the readtion. a ing of the watt meter will be directly dependent An investigation shows that the sources of error upon the geographical angle between the earth whenputting the cores in the east-west direcmagnetism and the longitudinal axis of the cores tion may be of two diflerent kinds. Iii-l 0. The dependence is expressed in the form The first of these special sources 01' error is due to the fact that the rate of modulation of the modulated signal according to Equation may be represented as In order to increase themaximumvalue of the rate of modulation the field intensity Hm should be very small in relation to the field intensity H11. A further improvement is achieved if the resistor I9 is connected between earth and the control grid of the tube 2| so that the direct current voltage drop across the resistor I9 will vary and thereby give the tube 2| 2. variable grid bias. The tube 2| should preferably be a so called variable mu tube, so that the variation of the amplification will counteract the said error.

The other one of the two special sources of .error I, which occur in the E-W position results from the creation of a second harmonic. If the rectifier I8 is not functioning on the square law a second harmonic of the low frequency component of the rectified voltage will be produced.

Now it is known that an idealsquare rectifier cannot. be -obtained, but'that all rectifiers have certain errors with respect to their-character istic's. This is especiallyjtrue'in the" case of a rectifier of the diodetypeor a contact. rectifier which functions with a relatively small square term in. their characteristics, the linearv term being the greatest although certain terms of higher degree are also present." In practice therefore it is not possible to prevent a Second harmqn'ic pathdueto the earths magnetic field.

the measuring voltage from being fed to. the watt meter by the simple arrangement of Fig. 1.

As a matter of fact this second harmonic should not necessarily be or any harm if it were not for the presence of the second harmonic of'the voltage from the line 30.. These two oscillations may cause an error in the reading of the watt meter in the E-W direction. a I

It is evident that harmonics of higher rate than the second could alsoprovide a similar eflect, but-in practice it has proved that such higher harmonics usually have a comparatively small .amplitude. Their influence, therefore is negligible.

The last mentioned disadvantage may be re moved according to a further embodiment of the invention by connecting a filter 33 in the line circuit 30. Fig. 1, and by providing a double acting rectifier as shown in Fig. 4.

The arrangement according'to Fig. 4 is meant to replace the part surrounded by the broken line 34 in Fig. 1, the connection points to other parts of Fig.1in this figure as well as in Fig. 4 being designed as 35 and 36. respectively. In other respects similar parts in Fig. 1 and Fig.- 4have been designated with similar reference characters.

6. In the arrangement according to Fig. 4 the oscillations from each of the windings l6 and II, respectively, are rectified separately in rectifiers 31 and 38. The load resistor is designated 39 and t the condenser for by-passing this load resistor at 40. It is known that such a symmetrical rectifier will produce no even harmonics.

The invention is, of course, 'not limited to the above embodiments, but certain modifications may be made without departing from the scope of the invention.

What I claim is: I 1. In an induction compass. a magnetic path sensitive to the earths magnetic field, comprising a pair of relatively stationary magnetic members and a rotatable magnetic member mounted to vary the reluctance of said path, means rotating the rotatable member ata uniform speed said relatively stationary members being mounted for orientation in the earth's field, windings on said last members connected to carry an induced voltage, a polarizing winding associated with said magnetic path, a source of alternating current connected toene'rgize said last windin for pro- 'ducing an alternating polarizing field, whereby said induced voltage represents a voltage due to said alternating polarizing-field modulated by the voltage due to the earth's field, a rectifier connected to rectify said inducedvolt'age and an alternating current watt meter having a pair of U windings connected respectively to said alternat- I ing current source and to the output of said rectifier.

2. An induction compass as set forth in claim 1 v in which said polarizing winding is energized to produce an alternating polarizing field which is small as compared to the field in said magnetic 3. In an induction compass as set forth in claim 1, phase shifting means connected between said alternating current source and said polarizing winding andhaving means to cause the field produced by said polarizing winding to be in phase with the voltage supplied to said watt meter by saidrectifier.

CARL-ERIK GRANQVISI.

REFERENCES crrnn, I The following references are of record in the file of this patent:

UNITED STATES PATENTS 2,334,469 Alexandersson et ai. Nov. 16, 1943 

